Is it because Mayer has operated Yahoo well, and investors are optimistic about the company's future?

Maybe. But not really.

The truth is, Yahoo's stock is exploding because Yahoo owns a very large stake in a pre-IPO Chinese e-commerce company called Alibaba, and that makes Yahoo is the easiest way for investors to bet on Alibaba.

That's good news for Yahoo, because Alibaba is doing extremely well.

In terms of its ambitions, it's more like a Chinese Google or a Chinese Amazon than a Chinese Yahoo.